The invention relates to copolymers of tetrafluoroethylene and “(perfluorobutyl)”ethylene produced by aqueous dispersion polymerization.
Many prior patents disclose techniques for the dispersion polymerization of tetrafluoroethylene, and variations thereof. The dispersion polymerization of tetrafluoroethylene produces what has come to be known as “fine powder” resins. In such a process, sufficient dispersing agent is introduced into a water carrier such that, upon addition of tetrafluoroethylene in the presence of a suitable polymerization initiator and upon agitation, and under autogenous tetrafluoroethylene pressure of 10 to 40 kg./cm2, the polymerization proceeds until the level of colloidally dispersed polymer particles is reached and the reaction is then stopped. See, e.g., U.S. Pat. No. 4,016,345 (Holmes, 1977).
Tetrafluoroethylene powders have also been produced by a process of suspension polymerization, wherein tetrafluoroethylene monomers are polymerized in a highly agitated aqueous suspension in which little or no dispersing agent may be employed. The type of powder produced in suspension polymerization is termed “granular” resin, or “granular powder”. See, e.g., U.S. Pat. No. 3,655,611 (Mueller, 1972).
For both types of polymerization processes, copolymerization of tetrafluoroethylene with various fluorinated alkyl ethylene comonomers has been described. See, for example, U.S. Pat. No. 4,792,594 (Gangal, et al., 1988). The present invention relates to the aqueous dispersion polymerization technique wherein the product of the polymerization reaction is the copolymer of the invention dispersed within an aqueous colloidal dispersion. This process, generally, is one in which tetrafluoroethylene monomer is pressured into an autoclave containing water and certain polymerization initiators along with paraffin wax to suppress coagulum formation and an emulsifying agent. The reaction mixture is agitated and the polymerization is carried out at suitable temperatures and pressures. Polymerization results in the formation of an aqueous dispersion of polymer. The dispersed polymer particles may be coagulated by techniques known in the art to obtain fine powder polymer. When fluorinated alkyl ethylene comonomers are introduced into the polymerization, it is known that the comonomer reacts much faster than tetrafluoroethylene monomer, and comonomer addition rate is important to the distribution of comonomer achieved in the copolymer. When this comonomer is added as a single precharge, the comonomer is found in polymerized form mostly in the core or interior of the polymer particles. The comonomer may also be injected through some or all of the polymerization process and the injection sequence determines the structure of the shell, i.e., if comonomer is added throughout, it will reside throughout the outer shell of each copolymer particle.
Various prior patents have disclosed variations on techniques for the homopolymerization of tetrafluoroethylene and for the copolymerization of other monomers with tetrafluoroethylene. Among those are included U.S. Pat. No. 4,576,869 (Malhotra, 1986) and U.S. Pat. No. 6,177,533B1 (Jones, 2001). Within those references are contained certain procedures which have become, more or less, accepted procedures for determining certain defining and delineating properties associated with tetrafluoroethylene homopolymers and copolymers. Among those properties are:
(a) the Standard Specific Gravity (SSG), measured by water displacement of a standard molded test specimen, in accord with ASTM D-1457-90;
(b) Raw Dispersion Particle Size (RDPS), determined by spectrophotometry or other suitable technique. See, e.g., U.S. Pat. Nos. 4,016,345 and 4,363,900. The measurements herein were obtained by laser light scattering using a Brookhaven 90 plus instrument.
In the cited prior patents, and almost universally, the SSG of a homopolymer specimen has come to define its molecular weight, with the relationship being inverse, that is, a high molecular weight (MW) corresponds to a low SSG and, generally, the lower the SSG, the higher is the molecular weight. Addition of comonomer into the polymerization process may also reduce SSG and, for resins modified with comonomer, SSG may be used to infer variations in molecular weight at a given constant comonomer level.
For fluoroethylene fine powder polymers, generally, their RDPS range from about 0.175 microns and below to about 0.325 microns. These fine powder resins are known to be useful in paste extrusion processes and in stretching (expansion) processes in which the paste-extruded extrudate, after removal of extrusion aid lubricant, is stretched rapidly to produce porous, strong products of various cross-sectional shapes such as rods, filaments, sheets, tubes, etc. Such a stretching process is disclosed in U.S. Pat. No. 3,953,566 (Gore, 1976), assigned commonly with the instant invention.
Heretofore it has generally been accepted that, for tetrafluoroethylene homopolymers and copolymers of the dispersion type, it is difficult to achieve a resin which combines both desirable properties of small raw particle size (RDPS) coupled with a high molecular weight (MW). Expressing the same conclusion in a different, equivalent way, it is generally accepted that a dispersion resin possessing a small raw dispersion particle size (RDPS) and a low standard specific gravity (SSG) has been difficult or impossible to achieve.
This invention provides a dispersion type copolymer of tetrafluoroethylene and perfluorobutylethylene comonomer which possesses a heretofore unachieved combination of small fundamental resin particle size (RDPS) coupled with a low SSG (high MW).